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. COLORADO LEGACY LAND <br /> 4601 DTC Boulevard, Suite 120, Denver, CO 80237 <br /> T. (303) 862-3928 <br /> May8,2019 <br /> Mr.Michael Cunningham and Ms.Amy Eschberger <br /> Division of Reclamation,Mining,and Safety <br /> Department of Natural Resources <br /> 1313 Sherman Street,Room 215 <br /> Denver,CO 80203 <br /> Subject: Technical Revision#28: Revised North Waste Rock Pile Drainage Design,Construction,and <br /> Schedule for Implementation <br /> Mine Land Reclamation Permit M-1977-300,Schwartzwalder Mine,Golden,Colorado <br /> Dear Mr.Cunningham: <br /> Colorado Legacy Land, LLC (CLL) would like to revise Mine Land Reclamation Permit M-1977-300 for the <br /> Schwartzwalder Mine,to include the following design,construction,and project schedule for a diversion pipeline <br /> on the North Waste Rock Pile (NWRP). The primary goal of this project is to intercept and divert stormwater <br /> from the ephemeral drainage above the NWRP,around the waste rock,and into Ralston Creek. <br /> The previous site owner,Cotter Corporation N.S.L. (Cotter) submitted a design in Technical Revision #23 for a <br /> diversion ditch. CLL is submitting Technical Revision #28, to revise this design and provide a schedule for <br /> construction. <br /> Drainage Design: <br /> Table 1 summarizes the key differences between the previous design approved in Technical Revision #23 and <br /> the current proposed design: <br /> Table 1. ©esi Sum air and C >isals <br /> crkvria Technical Revision#23 Teckdcal Revision#20 <br /> Design Flow 100- ear 67.7 cubic feet per second cfs 67.7 cfs <br /> Area of Watershed 0.059 square miles 0.059 square miles <br /> Aesthetics Above ground Below ground <br /> Intake Concrete ditch Concrete headwall <br /> Diversion Concrete and rip rap lined ditch Buried pipeline <br /> The hydrology used for the design is the same hydrology used for Technical Revision #23,which used the SCS <br /> Runoff Curve Number procedure. The SCS method predicts a 100-year runoff event will have a peak flow of 67.7 <br /> cubic feet per second(cfs). The primary difference between Technical Revision#23 and Technical Revision#28 <br /> are the aesthetics. <br /> Drainage Construction: <br /> The NWRP diversion pipeline consist of four primary components. Water from the ephemeral draining above <br /> the NWRP is intercepted prior to running onto the NWRP at a headwall. From the concrete headwall water enters <br /> a 30-inch pipe which runs adjacent to the NWRP access road. At the base of the NWRP, the water enters a <br /> manhole and flows into a 42-inch pipe before discharging into Ralston Creek. <br />